I am simply retrieving all the products that are listed on that particular website (they have multiple) and that's dynamic, so the filters are just all the MYOB Item Numbers. When it works it's way more efficeint than retrieving all the products in the MYOB database, as many products don't relate to that website.
However, you raise a good point: if my client was prepared to do an an extra step and mark all the products that relate to that website as a custom field then that should solve my problem. I can ask them.